1. Field of the Invention
The present invention relates to a signal detector, and more specifically, it relates to a signal detector such as a surface pressure input panel detecting a surface pressure or a two-dimensional image sensor.
2. Description of the Prior Art
An active matrix surface pressure input panel for detecting fine pressure distribution such as that of a finger pattern with a thin-film transistor or a thin-film diode is known as an exemplary signal detector. A two-dimensional image sensor for reading a two-dimensional image with a photo-receiving sensor element is also known as another exemplary signal detector. For example, Japanese Patent Laying-Open No. 6-62175 (1994) discloses such a two-dimensional image sensor.
FIG. 9 is a block diagram showing the circuit structure of the conventional two-dimensional image sensor disclosed in the aforementioned gazette. Referring to FIG. 9, pixels are arranged in the form of a matrix in the conventional two-dimensional image sensor. Each pixel is formed by a pixel switch (TM11 to TMnm) 1 consisting of an amorphous silicon TFT, an amorphous silicon thin-film photosensor 2 serving as an optical sensor and a storage capacitor (C11 to Cnm) 3.
A gate electrode of the pixel switch 1 is connected to a subscanning line (Hi: i=1 to n) 4 extending in a main scanning direction (X-direction). A subscanning circuit 5 applies a pulse voltage for successively turning on the pixel switch 1 to each subscanning line 4. A drain electrode of the pixel switch 1 is connected to a main scanning line (Vi: i=1 to m) 6 extending in a subscanning direction (Y-direction). The main scanning line 6 is connected to a horizontal switching transistor (THi: i=1 to m) 8 driven by a main scanning circuit 7. A drain electrode of the horizontal switching transistor 8 is connected to a common output line 9. The main scanning lines V1 to Vm have capacitances CV1 to CVm respectively.
In order to detect data in high precision, the conventional two-dimensional image sensor (signal detector) shown in FIG. 9 must increase the number of pixels by refining the respective pixels. When the number of the pixels is increased, however, peripheral circuits such as the main scanning circuit 7 and the subscanning circuit 5 driving the main scanning lines 6 and the subscanning lines 4 respectively are also increased in size. Therefore, the layout area for the peripheral circuits is disadvantageously increased while the yield is reduced in the peripheral circuit part.
Further, the conventional two-dimensional image sensor (signal detector) shown in FIG. 9 has only the single output line (signal read line) 9, and hence the reading time for fetching all data is disadvantageously increased when the number of the pixels is increased.